Abstract. The umbrella cells that line the inner surface of the bladder form an impermeable barrier that must accommodate large changes in urine volume as the bladder fills and empties. In addition, these cells in conjunction with the other cell types that comprise the uroepithelium function as an integral part of a sensory web. In this web, the uroepithelium responds to mechanical stimuli and mediators in the extracellular environment and transmits this information to the underlying nervous and muscular tissues. Exocytosis/endocytosis at the apical membrane of the umbrella cell is important for normal uroepithelial function because it regulates the barrier and sensory functions of the uroepithelium, in part, by modulating the membrane content of surface receptors and channels, other structural proteins such as the uroplakins, and membrane lipids. Furthermore, alterations in membrane traffic are likely to promote the invasion and release of uropathogenic bacteria, as well as contribute to the defective expression of surface receptors and other proteins that is associated with bladder diseases such as interstitial cystitis. The overall hypothesis to be tested In this proposal is that umbrella cells respond to their mechanical environment by dynamically regulating the insertion and/or retrieval of membrane at their apical plasma membrane. We previously observed that stretch stimulates both exocytosis and endocytosis. In Aim 1 we will explore the mechanisms of the stretch-induced endocytosis, the fate of internalized membrane, the signaling machinery that initiates this response, and the role of this pathway in modulating the exocytic response. In Aim 2 we will elucidate the mechanisms by which stretch triggers both rapid (early and slow (late) exocytic responses. This exploration will include the identification of the non-selective cation channel that triggers the early response, and the upstream mechanisms that lead to autocrine activation of the epidermal growth factor receptor, which modulates the late exocytic response. In addition, we will define the role of synaptotagmin VII and Rablla in these trafficking events. In Aim 3 we will explore the mechanism of voiding-induced endocytosis and examine the fate of the internalized membrane components. Furthermore, labeling of internalized surface membrane and inhibition of vesicle synthesis will be used to determine if post-voiding replenishment of DFV occurs as a result of de novo synthesis. These studies will provide a better understanding of how the umbrella cell adjusts its apical membrane content and surface area in response to extracellular stimuli such as membrane stretch.